Antibodies such as bevacizumab targeting the VEGF-VEGFR-2 pathway of tumor angiogenesis have little or no impact in the treatment of advanced metastatic disease. Instead, clinical benefit is sometimes attained when used in combination with standard chemotherapy regimens. However, even in such cases, the survival benefits are modest. The basis for the chemosensitizing ability of antiangiogenic therapy remain unclear; improvements are likely to come from a better understanding of the mechanisms involved. Hypothesis: Conventional chemotherapy can, in some cases, induce an acute mobilization of bone marrow (BM)-derived cells, including VEGFR-2+ circulating endothelial progenitors (CEPs) which home to and colonize drug treated tumors in large numbers, thus altering the tumor angiogenic microenvironment and stimulating tumor regrowth; this systemic host response can be significantly blocked by co-treatment with a drug such as VEGFR-2 targeting antibodies, or low-dose `metronomic' (antiangiogenic) chemotherapy. Specific Aims: First, a number of chemotherapy drugs will be tested for induction of acute mobilization and tumor homing of CEPs, and whether among those that do, their anti-tumor efficacy is increased by blockade of this host response, using both pharmacologic and genetic approaches. The second aim involves an analysis of chemotherapy-induced BM-derived cell colonization of orthotopic versus ectopic primary transplanted tumors as well as advanced metastases growing in the liver, lungs, or brain. The third aim is devoted to analyzing molecular mechanisms mediating chemotherapy-induced BM-derived cell mobilization and tumor colonization including role of G-CSF, a41 integrin and the CXCR4 chemokine receptor and its ligand SDF-1. The fourth aim will evaluate the contribution of other BM-derived pro-angiogenic CD45+ cell populations having `vascular leukocyte properties', e.g. Gr1+CD11b+ myeloid cells or tie-2 expressing monocytes, among others. Finally, the fifth aim is designed to assess whether surges in CEPs are induced in cancer patients by chemotherapy, including MTD taxanes, and if so, whether bevacizumab blunts such host responses. Significance/Impact: The research links chemotherapy, tumor angiogenesis, antiangiogenic therapy, and the tumor microenvironment in a new way; it will indicate new targets to consider for anti-cancer therapy when combined with chemotherapy, but which may vary with the metastatic environment, and provide new perspectives for the contribution of BM-derived cell populations, including CEPs, in tumor biology and therapy.